1. Field of the Invention
The present invention is directed to very high output (VHO) lamps having a lamp envelope with phosphor coating, and more particularly, to a tri-phosphate coating over an alumina pre-coat and a long mount electrode coated with alumina.
2. Discussion of the Prior Art
Low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and rare gas to maintain a gas discharge during operation. The radiation emitted by the gas discharge is mostly in the ultraviolet (U.V.) region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation.
There is an increase in the use of fluorescent lamps because of reduced consumption of electricity. To further reduce electrical consumption, there is a drive to increase efficiency of fluorescent lamps, referred to as luminous efficacy which is a measure of the useful light output in relation to the energy input to the lamp, in lumens per watt (LPW).
To this end, different blends of phosphors are used for the luminescent coating. Further, a metal oxide layer is provided between the luminescent coating and glass envelope. The metal oxide layer reflects the U.V. radiation back into the phosphor luminescent layer through which it has already passed for further conversion of the U.V. radiation to visible light. This improves phosphor utilization and enhances light output. The metal oxide layer also reduces mercury consumption by reducing mercury bound at the tubular portion of the lamp.
To further reduce mercury consumption, the glass seals supporting the electrodes at both ends of the lamp are coated with the metal oxide layer to reduce mercury bound at the end portions of the lamp.
The conventional fluorescent lamps described above typically operate at low power levels, such as 40 watts. Conventional 8 foot VHO lamps with high wall loading can operate on a current of 1.5 Amps, with a lamp power of 215 Watts. Conventional VHO lamps are made with a single layer of phosphor and are manufactured with approximately 15 to 40 mg of mercury. There is a need for a fluorescent lamp with high wall loading, operating efficiently at power levels greater than 100 watts with minimal mercury consumption.
The object of the present invention is to provide a very high output (VHO) fluorescent lamp with increased luminous efficacy and reduced mercury consumption.
The present invention accomplishes the above and other objects by providing an electric lamp having an envelope with an inner surface and two electrodes located at ends of the electric lamp. The electrodes generate ultraviolet radiation in the envelope which is filled with mercury and a charge sustaining gas.
The inner surface of the envelope is pre-coated with an aluminum oxide layer to reflect ultraviolet radiation back into the envelope. A tri-phosphate layer is formed over the aluminum oxide to convert the ultraviolet radiation to visible light. The tri-phosphate layer consists of yttrium oxide, cerium magnesium aluminate, and barium-magnesium aluminate.
One of the electrodes is mounted on a short mount along with a mercury capsule, while the other electrode is mounted on a long mount. The long mount has a horizontal portion and a flared portion which is near the lamp end. The horizontal portion is coated with a layer of aluminum oxide to reduce mercury consumption.